I. Field of the Invention
The present invention relates to communication. More particularly, the present invention relates to a novel and improved method and apparatus for providing communication over a digital link.
II. Description of the Related Art
A variety of apparatus exists today that use standard telephone networks to convey data from one terminal to another. One of the most common terminals that uses a standard telephone network to convey data is the facsimile (FAX) machine. Like other terminal equipment that use standard telephone networks, a FAX machine uses a modem to translate digital information to audible analog tones for transmission over the telephone network. At the receiving FAX machine, a modem is used to convert the tones back into an estimate of the original digital information sent by the transmitting FAX machine.
With the deployment of cellular and personal communications wireless systems, the end user may wish to connect his terminal equipment to a mobile communications device instead of directly to a land telephone network. However, if the user attempts to send the audible tones produced by a modem over a standard analog FM cellular channel, it is doubtful that he will be successful. In general, the FM cellular channel introduces a substantial amount of noise into the link compared to the standard land telephone network. The additional noise translates into errors in the estimation of the digital information when the signal is converted at the receiving end. The errors on the link can be overcome by constraining communications to very low transmission rates.
If the end user attempts to send the audible tones produced by a modem over a standard digital wireless channel, it is also doubtful that he will be successful. Digital wireless communication equipment typically uses vocoders to convert incoming voice signals into digital bits for transmission over the channel. Vocoders are tailored to sample and compress human voice. Because modem tones differ significantly from human voice, the vocoder can cause noticeable degradation to the modem tones. Also, maximizing the capacity of the system in terms of the number of simultaneous users that can be handled is extremely important in a system using a wireless link. Digitizing modem tones with a vocoder and sending them over the digital wireless link is an inefficient use of the radio channel.
A more efficient, flexible, and reliable transmission means is to provide a mechanism to transmit the digital data over the digital wireless link directly. This configuration presents some unique opportunities to take advantage of the digital link to provide high quality service.
A typical digital wireless link is not an error free channel. To provide error free communication, the wireless link protocol may provide re-transmission of lost or corrupted data. The re-transmissions introduce arbitrary and significant delays in the data. When the time sensitive messages are transmitted over the wireless link, the timing of the signals may be lost thus prohibiting proper communications.
It is therefore the object of the present invention to provide an efficient method and apparatus for communication with standard terminal equipment over a digital wireless link.
It is a further object of the present invention to provide a means of communicating time sensitive information over a variable time delay link.